1. Field of the Invention
The present invention relates to an electrodeposition apparatus and an oxide film forming method for forming an oxide film by electrodeposition (which includes electrolytic plating and electrolytic deposition, both having the same meaning) on a long substrate such as a stainless steel belt-like plate, and more particularly to an electrodeposition apparatus and an oxide film forming method for uniformly forming a zinc oxide film on the substrate.
2. Related Background Art
In the production of photovoltaic elements, the technology of forming oxide on a substrate by an electrochemical reaction in an aqueous solution is being investigated as an atternative the vacuum process. For example, the Japanese Patent Laid-Open Application No. 09-092861 proposes xe2x80x9ca method of producing a photovoltaic elementxe2x80x9d and discloses xe2x80x9ca method of forming an oxide such as zinc oxide on a long substrate by electrodeposition and an apparatus thereforxe2x80x9d.
FIG. 2 is a schematic view showing an example of the apparatus for forming an oxide by electrodeposition, which was prepared by modifying the apparatus described in the above-mentioned patent application by the present inventors, and simplified to function only to form a zinc oxide film by electrodeposition. The apparatus shown in FIG. 2 does not constitute the prior art.
In FIG. 2, a long substrate 2001 of stainless steel is fed from a rolled stainless steel sheet, which is also called a hoop material, a roll-type substrate or a web. The long substrate 2001 is transported to the apparatus in the state of a coiled substrate, wound on a bobbin.
In the present apparatus, the coiled long substrate 2001 is mounted on a substrate feeding roller 2002. While an interleaving paper inserted for surface protection is wound up on a interleaf wind-up roller 2003, the substrate is transported as indicated by an arrow 2004 toward a substrate wind-up roller 2062 and wound thereon as indicated by an arrow 2061.
In the transporting path, the substrate 2001 passes through a tension detecting roller 2005 and an electric power supplying roller 2006 an enters and electrodeposition vessel 2009. In the vessel 2009, the substrate 2001 is positioned by supporting rollers 2013 and 2014 and is subjected to the formation of an oxide film by electrodeposition.
After passing the electrodeposition vessel 2009, the substrate 2001 is introduced into a rinsing vessel 2030 and washed therein. The substrate is positioned in the rinsing vessel 2030 by supporting rollers 2031 and 2066. After passing the rinsing vessel 2030, the substrate 2001 is introduced into a hot air drying oven 2051 and dried therein.
After passing the drying oven 2051, the substrate 2001 passes through a supporting roller 2057, then is subjected to the correction of lateral displacement by a skew correcting roller 2059 and is wound on the substrate wind-up roller 2062 together with a new interleaving paper supplied from an interleaf feeding roller 2060. Then the substrate is transported to a next step when required.
The tension detecting roller 2005 detects the dynamic tension of the substrate 2001 and applies a feedback to braking means such as a powder clutch (not shown in the drawings) linked to the shaft of the substrate feeding roller 2002, thereby maintaining a constant tension. Thus, the transporting path of substrate 2001 is designed so as to have a predetermined tension value between the supporting rollers.
In particular, in the present apparatus in which the film forming surface of the substrate is free from contact with the rollers, weak tension may result in various defects such as disengagement of the substrate 2001 from the supporting rollers or hanging of the substrate 2001 at the entrance or exit of the electrodeposition vessel 2009 or the rinsing vessel 2030, leading to the damage on the substrate by frictional contact thereof. Consequently the tension detecting roller 2005 is an important member.
The configuration in which the apparatus does not contact the film forming surface has the advantages that the film forming surface is free from damage or smear and is particularly advantageous for an application in which irregularities in size on the order of several microns have to be formed on a thin film, as in the case of the reflection film of a solar cell.
An electric power supplying roller 2006 for serving to supply a cathodic potential to the long substrate is positioned as close as possible to the electrodeposition bath and is connected to the negative electrode of a power source 2008.
The electrodeposition vessel 2009 serves to hold the electrodeposition bath 2016, to determine the path of the substrate 2001 and to support an anode 2017 so as to oppose to the substrate 2001. The anode 2017 is connected via an electric power supplying bar 2015 to the positive electrode of the power source 2008, and a positive voltage is applied to the anode 2017. Thus there is executed an electrochemical electrodeposition process in the electrodeposition bath, with the substrate 2001 as the negative side and the anode 2017 as the positive side.
When the electrodeposition bath 2016 is maintained at a high temperature, there is generated a considerable amount of water vapor and vapor exhausting dusts 2010, 2011 and 2012 are used to remove the water vapor.
Also, in order to agitate the electrodeposition bath 2016, air is introduced from an agitating air introduction pipe 2019 and bubbling is executed by blowing air from an air blow pipe 2018 in the electrodeposition vessel 2009.
For supplying a high temperature bath to the electrodeposition vessel 2009, a heater 2024 is provided in an electrodeposition circulating vessel 2025 for heating an electrodeposition bath solution and the heated solution is supplied to the electrodeposition vessel through a circulating pump 2023 and an electrodeposition bath solution supply pipe 2020. The solution overflowing from the electrodeposition vessel 2009 and a part of the solution to be positively circulated are returned through a feedback path (not shown in the drawings) to the circulating vessel 2025 and are heated again therein.
In case the discharge amount of the pump 2023 is constant, the amount of the solution supplied from the circulating vessel 2025 to the electrodeposition vessel 2009 can be controlled by valves 2021 and 2022. In order to increase the supply amount, the valve 2021 is opened more while the valve 2022 is closed more, and vice versa. The level of the electrodeposition bath 2016 is adjusted by such a supply amount and the feedback amount from the feedback bath.
The circulating vessel 2025 is provided with a filter circulating system consisting of a circulating pump 2027 and a filter, in order to remove particles in the circulating vessel 2025. If the supply and feedback amounts of the solution between the circulating vessel 2025 and the electrodeposition vessel 2009 are sufficiently large, satisfactory removal of the particles can be achieved by positioning the filter solely in the circulating vessel 2025.
In the present apparatus, the circulating vessel 2025 is also provided with a vapor exhausting duct 2026 for removing the vapor. Since the circulating vessel 2025 constitutes a heat source by arranging the heater 2024 therein, there is generated a significant amount of vapor and such vapor removal is very effective in case unexpected release or condensation of thus generated vapor is undesirable.
An electrodeposition reserve vessel 2029 is provided for preventing damage to a solution disposing unit which is caused by direct discharge of the heated solution to the solution disposing system, and can hold the electrodeposition bath 2016 of the electrodeposition vessel 2009 when the valve 2028 is opened, thereby evacuating the electrodeposition vessel 2009 and improving the efficiency of the work therein.
The substrate 2001 subjected to electrodeposition in the electrodeposition vessel 2009 is then introduced into the rinsing vessel system 2030 and washed therein. In the rinsing tank system 2030, the substrate 2001 is positioned by the supporting rollers 2031 and 2066 and passes the first rinsing vessel 2032, the second rinsing vessel 2033 and the third rinsing vessel 2034 in succession.
The rinsing vessels 2032 to 2034 are provided with water circulating vessels 2047 to 2049 and water circulating pumps 2044 to 2046, respectively. The water supply amounts to the rinsing vessels are determined by two valves 2038 and 2041; 2039 and 2042; and 2040 and 2043, respectively, and rinsing water is supplied into the rinsing vessels 2032 to 2034 through water supply pipes 2035 to 2037, respectively.
The control of the supply amount with two valves is similar to that in the electrodeposition vessel 2009. Also an overflowing water or feedback water (not shown in the drawings) which is a part of water to be positively returned may be returned to the circulating vessels 2047 to 2049 similarly to the case of the electrodeposition tank 2009.
In the three-step rinsing system as shown in FIG. 2, the rinsing water generally becomes clearer from the rinsing vessel at the upstream side in the substrate transporting direction toward than at the downstream side, namely from the first rinsing vessel 2032 toward the third rinsing vessel 2034. This indicates that the substrate 2001 clearer as it is transported toward the end of the rinsing process.
Based on this fact, the water usage can be significantly saved by feeding the rinsing water at first to the third rinsing vessel 2049, then feeding the water overflowing therefrom to the second rinsing vessel 2048 and feeding the water overflowing therefrom to the first rinsing vessel 2047.
The substrate 2001 after rinsing is subjected to removal of water by an air knife 2065 provided in a part of the rinsing vessel system 2030 and is then transported to the hot air drying oven 2051, in which drying is executed with convection air having a temperature sufficient for evaporating water. The convection air flow is supplied by blowing hot air which is generated in a hot air generating oven 2055 and passed via a filter 2054 for particle removal, from a hot air blow pipe 2052.
The overflowing air is recovered from a hot air recovery pipe 2053 and returned to the hot air generating oven 2055 after mixing with external air introduced from an external air introduction pipe 2056.
The transporting path of the substrate 2001 in the hot air drying oven 2051 is positioned by the supporting rollers 2066 and 2057.
The skew correction roller 2059 corrects the aberration of the substrate 2001 in the width direction of the substrate and feeds the substrate for winding on the wind-up roller 2062. The roller 2059 is controlled by rotating about an arm (not shown in the drawings) based on the amount of aberration detected by a sensor not shown in the drawings. Ordinarily the amount of aberration detected by the sensor and the amount of actuation of the skew correcting roller 2059 are both quite small and do not exceed 1 mm.
In winding up the substrate 2001, a new interleaving paper is supplied from the interleaf feed roller 2060 in order to protect the surface film.
Stoppers 2007 and 2058 function at the same time to support the substrate 2001 standstill under a tension in order to improve the work efficiency at the replacement of the substrate 2001 or at the maintenance of the apparatus.
The electrodeposition apparatus shown in FIG. 2 provides the following advantages.
Firstly, the film formation is much simpler in comparison with that in a vacuum apparatus such as a sputtering apparatus. There is not required an expensive vacuum pump, and there is also not required designing of the power source or electrode for using plasma.
Secondly, the running cost is lower in most cases. This is because the sputtering apparatus requires an expensive target due to its preparation using manpower and an additional apparatus and the efficiency of target utilization of generally 20% or lower. Therefore, the target replacement work is frequently conducted to reduce productivity in case the throughput of the apparatus or the film thickness is large.
It is also superior in the apparatus cost and the running cost of other methods such as CVD or vacuum evaporation.
Also, the formed film is composed of polycrystaline fine particles in most cases. It is comparable in the electrical conductivity and in the optical characteristics to the film obtained in the vacuum method and superior to the film obtained by the sol-gel method, coating method utilizing organic substances or spray pyrolysis method.
Furthermore, other than these advantages are also achieved in forming an oxide, such as ease of disposal of the used solution, little influence on the environment and a low cost required for preventing environmental pollution.
However, the film formation with the above-described electrodeposition apparatus has resulted in the following drawbacks.
More specifically, though the electrodeposition in the electrodeposition vessel 2009 is satisfactory, the formed film surface dries before reaching the rinsing vessel system 2030 because in the high solute concentration of the electrodeposition bath 2016 and the high bath temperature, thereby generating unevenness in the form of a vague undulating stripe pattern on the formed film surface after drying. For example, in the production of a solar cell such unevenness in the form of vague undulating stripe pattern still remains even after formation of a semiconductor electromotive layer composed mainly of amorphous silicon by CVD and of a transparent conductive layer such as ITO, and results in uneven portions.
The present invention is to provide an electrodeposition apparatus capable of uniformly electrodepositing a uniform oxide film on a substrate, thereby obtaining an oxide film suitable for, for example, the reflective layer of the solar cell.
The present invention is to provide an electrodeposition apparatus comprising at least one electrodeposition vessel for supplying a current between a substrate and an electrode in an electrodeposition bath to form an oxide film on the substrate and a rising means for rinsing the substrate with water after passing the electrodeposition vessel, wherein a humidifying means to prevent drying of at least the film forming surface of the substrate is provided along the transporting path of the substrate at least at the exit side of the electrodeposition vessel.
Also the present invention is to provide an electrodeposition apparatus comprising at least one electrodeposition vessel for supplying a current between a substrate and an electrode in an electrodeposition bath to form an oxide film on the substrate and a rising means for rinsing the substrate with water after passing the electrodeposition vessel, wherein the substrate after the is then transported into another electrodeposition vessel or a rinsing means before the substrate surface is dried spontaneously to deposit the solute in the electrodeposition bath.
Further, the present invention is to provide a method of forming an oxide film, which comprises supplying a current between a substrate and an electrode in an electrodeposition bath of at least one electrodeposition vessel to form an oxide film on the substrate and rinsing the substrate after passing the electrodeposition vessel with water by a rinsing means, wherein at least the film forming surface of the substrate is humidified by a humidifying means along the transporting path of the substrate at least at the exit side of the electrodeposition vessel.
Furthermore the present invention is to provide a method of forming an oxide film, which comprises supplying a current between a substrate and an electrode in an electrodeposition bath of at least one electrodeposition vessel to form an oxide film on the substrate and rinsing the substrate after passing the electrodeposition vessel with water by a rinsing means, wherein the substrate after passing the electrodeposition vessel is transported to another electrodeposition vessel or a rinsing means before the substrate surface is dried spontaneously to deposit the solute in the electrodeposition bath.
By maintaining the substrate in a superhumidified state with such humidifying means, the solution transferred from the electrodeposition vessel to the substrate surface is prevented from drying, and the substrate in the humidified state is transported into the rinsing vessel, whereby the solute is not deposited to generate the unevenness of the film forming surface.
When the temperature of the electrodeposition bath is higher than room temperature, the transporting path between the electrodeposition vessel and the rinsing means may be covered by the enclosing means to maintain the interior thereof in a superhumidified state, whereby drying of the substrate resulting from the elevated substrate temperature can be prevented.
Such enclosing means may be composed of a water jacket to lower the ambient temperature, whereby the present invention becomes applicable even to an oxide sensitive to the temperature.
Also when several rinsing vessels are provided as the rinsing means and the temperature of the rinsing water is high to easily dry the substrate surface in the transporting path, the above-described humidifying means is provided between the rinsing vessels. The humidifying means can prevent formation of unevenness during film forming, and it also can enhance the rinsing effect by maintaining the rinsing water at a high temperature even when the electrodeposition bath contains a saccharide such as sucrose.
The humidifying means (liquid scattering means) includes a liquid mist spraying device, a device for generating small liquid droplets by utilizing the vibration of an ultrasonic vibrator, a humidifier utilizing a piezoelectric element, or a water vapor generating device.
When the humidifying means is composed of a device for generating small liquid droplets by utilizing the vibration of an ultrasonic vibrator, it is preferable that the vibration surface of the ultrasonic vibrator is inclined with respect to the film forming surface of the substrate and that the vibrator is provided with an ultrasonic power source for generating an ultrasonic wave in synchronization with the transportation of the substrate, liquid supply means for supplying a humidifying liquid to the vibration surface of the vibrator, liquid recovery means for recovering the humidifying liquid not formed into liquid droplets, and the circulating means for circulating the humidifying liquid from the liquid recovery means to the liquid supply means.
The humidifying liquid is preferably composed of water, a solution of the same composition as that of the preceding electrodeposition bath or a mixture of the solution and water.
In order to prevent metal deposition in the electrodeposition bath, it is preferably maintained at 60xc2x0 C. or higher.
In particular, the electrodeposition bath of 80xc2x0 C. or higher enables stable film formation in an electrodeposition process requiring a high temperature for zinc oxide or the like.
Also when the substrate is composed of a ferromagnetic material, magnet rollers in contact with the upper surface of the substrate as the transporting means are used to avoid vertical movement thereof in the overflowing portion, thereby preventing formation of the unevenness resulting from liquid overflowing or liquid splashing.
Furthermore, extremely stable formation of a zinc oxide film by electrodeposition can be executed when the oxide film formed in the electrodeposition bath is zinc oxide, the solution medium is water and the solute to be deposited by drying is zinc nitrate.
For achieving uniform oxide film formation, the electrodeposition bath preferably contains a saccharide.
Furthermore, with the transporting means composed of the rollers in contact with the substrate surface, the transporting direction of the substrate is not limited to horizontal, significantly broadening the freedom in designing the entire electrodeposition apparatus.